The invenlion relates to a microtome, in particular, an ultramicrotome.
In a known microtome, for example, that disclosed in West German Pat. No. 2,246,853, a specimen arm can be used to execute an oscillating-stroke movement by means of an electric-drive motor and crank mechanism. A drive-control system for the drive motor subdivides the stroke movement of the specimen arm into a comparatively slow-cutting movement, during which the specimen arm is guided past a knife to produce a thin section of the specimen present on the arm, and a more rapid return movement, during which the specimen arm is guided back, outside the knife, into its starting position.
As a rule, microtomes of the abovementioned type have a direct-current drive motor, coupled to a tachometer generator, and the motor drives the crank of the specimen arm via a drive belt and pulley which serves as a flywheel. The subdivision into a comparatively slow-cutting movement and a rapid return movement is effected in order to return the specimen arm, as rapidly as possible, to its starting position, following the cutting movement which is appropriately adjusted so as to produce optimum thin sections, this mode of operation being embodied in the interests of time saving. The two movements are controlled by reducing the control voltage appropriately at the transition from the comparatively rapid movement to the slower movement, and increasing the control voltage again at the transition to the more rapid movement.
Since, in order to achieve isolation from externally imposed disturbances caused by vibration, the drive mechanisms of microtomes, and especially of ultramicrotomes, incorporate resilient elements, the drive shaft of the drive motor starts to oscillate during the abovementioned speed change which causes the belt drive and the flywheel to vibrate as well. In such a case, the drive motor, which is controlled by means of the tachometer generator, can also be induced to resonate by the tachometer generator. Vibrations of this nature are, however, disadvantageous because they correspondingly manifest themselves in a vibratory movement of the specimen arm, this movement having an adverse effect on the quality of the thin section which is to be produced. In the case of the known microtomes, it is accordingly necessary to set the change-over from the comparatively-rapid return movement to the slower-cutting movement at a point in time which is sufficiently in advance of the start of the actual cutting operation to ensure that the vibrations, generated in the mechanical-drive system by the speed-change jerk, have already died away. This, however, reduces the rate at which the microtome operates.